Hi Simon!
I think with lilypond you wouldn't go wrong. You can convert it to MIDI (to
an extend) and you can make sheet music of it.
For ABC: I know you can convert it to MIDI, I don't know about sheet music
and human-readability.
For Lilypond I believe there are also graphical editors making it easier
for you. But don't nail me on this. :-)
Kindest regards
Julien
--------
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